In the manufacture of paper and paper board, it is well-known to incorporate quantities of inorganic fillers into the fibrous web in order to improve product quality. It is also well known to coat paper and paper board with inorganic pigments to improve opacity, print quality, and frequently sheet brightness. Titanium dioxide is widely used to improve brightness and opacity of coated and filled sheets but it is an expensive pigment. In recent years, considerable efforts have been made to develop satisfactory replacements for titanium dioxide. Substantially anhydrous kaolin clays prepared by partially or fully calcining a fine particle size fraction of crude kaolin clay is now a replacement pigment of choice. See, for example, U.S. Pat. No. 3,586,523, Fanselow et al, the teachings of which are incorporated herein by cross-reference. Calcined kaolin clay opacifying pigments, such as the products supplied under the registered trademarks ANSILEX and ANSILEX 93 by Engelhard Corporation are exemplary. These products are substantially anhydrous white pigments and are widely used as fillers in paper sheets and paper board, as a coating pigment for paper, and as a pigment in paints and other filled systems. They consist of aggregates of clay particles, and exhibit exceptionally high light scattering and opacifying characteristics when incorporated as a filler into paper. The particle size of these pigments is typically at least 65 percent by weight finer than 2 micrometers equivalent spherical diameter (esd), and at least 50 percent by weight finer than 1 micrometer. Especially when used to coat or fill paper products, low abrasion is essential. The calcined kaolin pigments presently used by the paper industry exhibit low abrasion values, generally less than 50 mg., and usually below 30 mg., as measured by the Valley test method.
Calcined kaolin clay pigment includes kaolin clays which have been dehydroxylated by heating to temperatures over 400.degree. C. The term "calcined" thereby embraces fully calcined kaolins-which usually have been heated above the 980.degree. C. exotherm, as well as so-called "metakaolin," which results from heating to lower temperatures, below the exotherm. Reference is made to the following which relate to fine particle size, low abrasion, opacifying calcined kaolin pigments, Proctor, U.S. Pat. No. 3,014,836; Fanselow et al., U.S. Pat. No. 3,586,823; Morris, U.S. Pat. No. 3,519,453; Podschus, U.S. Pat. Nos. 3,021,195 and 3,309,214, and British Pat. No. 1,181,491 and U.S. Pat. No. 4,381,948, McConnell et al.
Naturally-occurring sources of kaolin, generally known as kaolin crudes, such as those mined in Georgia, U.S.A. and Cornwall in England, are processed to recover upgraded kaolin products. Many Georgia crudes contain well-crystallized finely divided particles of kaolin having a median particle size of about 1.5 micrometers in association with silica and silicate impurities as well as colored ferruginous and titaniferous impurities. Other commercially important Georgia crudes contain less well-crystallized kaolin which are finer in particle size, e.g., median size below 1 micrometer. The latter find widespread use in calcined form in applications such as paper coating and filling in which high opacification and low abrasivity, frequently also high brightness, are demanded. Vast tonnages of such kaolin crudes are mined for the ultimate recovery of purified grades of kaolin having a higher brightness and different particle size distribution than that of the kaolin in the crude. The processing, frequently referred to as "wet processing", almost invariably involves crushing the kaolin crude, pulping the crushed crude in water, removing coarse impurities (so-called grit), fractionating the degritted crude to recover one or more fractions of desired particle size distribution, optionally bleaching to brighten one or more of the fractions of kaolin, filtration and washing. In some cases, additional upgrading to remove colored impurities by flotation, selective flocculation and/or magnetic purification is practiced and usually takes place before filtration and drying.
The manufacture of low abrasion calcined kaolin opacifying pigment has heretofore invariably utilized anionic wet processing of carefully selected crudes by processes including degritting, fractionation, recovery of the fine fraction (see, for example, Fanselow et al and McConnell patents, supra). In some cases, flotation, magnetic purification, bleaching or combinations of such steps is practiced. The recovered fine fraction of a selected crude, typically one composed of poorly crystallized kaolin, is recovered, pulverized, calcined and repulverized. Other low abrasion calcined pigments utilize as calciner feed material generally known as mechanically delaminated clay, a material recovered by mechanical delamination of coarse particle size fractions of well-crystallized kaolin recovered during wet processing. See, for example, Morris et al, supra.
When kaolin crudes are mixed in water without addition of dispersant the resulting pulps are usually mildly acidic, typically having a pH of 4-6 and the kaolin particles are flocculated and negatively charged. To the best of our knowledge, one or more anionic dispersants are invariably added to a pulp of the crude to create near neutral or mildly alkaline systems before degritting and fractionation and other wet processing steps such as delamination, flotation, magnetic purification, etc.
The history of using alkali or negatively charged material as primary dispersants for negatively charged clay dates at least back to 1912 (Reissue U.S. Pat. No. 14,583, Scherwin). This patent teaches using "electrolytes containing ions of an electric character which is the same as the character of the part of the composite mass which it is desired shall remain in suspension in sol-condition". This concept is pursued in Scherwin's subsequent patent (U.S. Pat. No. 1,233,713), which discloses fractional sedimentation of kaolin in a dispersed system. An advance is reflected in U.S. Pat. No. 1,324,958 (Feldenheimer) in which an acidic material is used to neutralize the alkaline deflocculating agent and to flocculate the clay in order to settle materials not ordinarily settled in a deflocculated condition. Subsequent patents disclose particle size fractionation of kaolin in dispersed state in centrifuges, usually followed by flocculation by acid addition, filtration and water washing of the filter cake. A major advance in the development of the kaolin industry was reflected in the establishment of a 2 micrometers particle size cut-off point when fractionating kaolins to prepare paper coating clay and the use of centrifuges to carry out the desired fractionation of the crude clay. Crudes having a median particle size of 1.5 micrometers are conventionally classified in centrifuges to recover premier coating grades (at least 90% by weight finer than 2 micrometers) and coarse filler kaolin having a median size of 4-6 micrometers. A major present use of kaolin crudes is as a source of bleached less than 2 micrometers fractions of kaolin clay supplied principally for the paper coating industry.
The growth of the kaolin industries in the U.S. and abroad has also generated many other innovations including the use of a variety of primary dispersants, all anionic, and including sodium silicates, silicate hydrosols, condensed phosphate salts, polyacrylate salts, and "auxiliary" dispersants such as sulfonates.
Cationic surfactants have been disclosed as a means to disperse various clays, but the utility is confined to dispersions in organic liquids. See, for example, U.S. Pat. No. 2,797,196, Dunn et al.
In accordance with U.S. Pat. No. 3,804,656, Kaliski, et al, a negatively charged filler or pigment such as kaolin is dispersed with a combination of nonionic and cationic surface active agents at a strongly alkaline pH. The cationic material is not added as a primary dispersant nor is it present during wet processing.
U.S. Pat. No. 4,738,726, Pratt et al, discloses high bulking pigments obtained by adding a cationic polyelectrolyte to previously degritted and fractionated kaolin clay to partially flocculate the clay. Anionic primary and secondary dispersants are used and the cationic material is used to partially flocculate, not to disperse the clay.
U.S. Pat. No. 4,210,488 (Reus et al) discloses addition of a cationic polymer (polypiperidine halide) "absorbed" on a fine particle carrier such as kaolin to improve the dry strength of paper or to improve the effect of optical brighteners. The absorbed polymer is said to create a positive charge on the kaolin particles. The cationic material is not added during wet processing of the carrier.
Numerous publications make reference to the addition of cationic polyelectrolytes to paper coating compositions containing mineral pigments such as clay or calcium carbonate to improve the coating structure by imparting a bulked structure. See, for example, Coco, C. E., Soy Protein Latex Interpolymers--Properties and Function, Preprints TAPPI Coating Conference 1987.
EPA 281,134 (Weige) relates to cationic pigment dispersion, especially for producing paper coating compositions, containing a pigment component, a cationised polymer, which encloses the pigment particles as a protective colloid, and optionally a cationic polymer and/or ammonium compound as dispersant for the coated pigment particles. The cationic polymer PVA increases the zeta potential of the dispersion to the isoelectric point or into the cationic region. The pigment can be ground or precipitated calcium carbonate, (calcined) kaolin, TiO.sub.2, ZnO, satin white, aluminum hydrosilicates or their mixtures.
U.S. Pat. No. 4,078,947, Bundy et al, relates to a process for producing a calcined clay product having finer particles and producing higher void volume in coatings wherein a Georgia kaolin is blunged with an anionic dispersant (sodium polymetaphosphate), fractionated and the fine fraction is treated with citric acid and polyamine fatty acid amine or a quaternary ammonium salt, flocculated, filtered, dried, pulverized, calcined and repulverized.
Our copending patent application, U.S. Ser. No. 07/309,456 filed Feb. 10, 1989, discloses the use of cationic polymers and the like as secondary dispersants to prepare stable aqueous slurries of calcined kaolin pigments.
To the best of our knowledge, the prior art does not disclose or suggest the use of cationics as primary dispersants for kaolin, much less for wet processed kaolin intended to be utilized as calciner feed to produce calcined kaolin pigments.